This invention relates generally to light amplifier tubes and is concerned more particularly with image intensifier tubes utilized for direct viewing of objects illuminated by visible or invisible radiation.
An image intensifier tube is a device for converting a radiational image of an external object directly into a bright visual image. Generally, an image intensifier tube comprises a tubular envelope having an input screen assembly disposed adjacent a radiation transparent faceplate at one end of the envelope and an imaging screen assembly disposed adjacent an output faceplate at the other end. The input screen assembly usually includes a transparent film of conductive material which serves as the cathode electrode and a superimposed layer of photoemissive material which functions as the photocathode of the tube. The imaging screen assembly generally includes a layer of phosphor material which functions as the imaging screen anode and an overlying film of conductive material which serves as the anode electrode of the tube. Usually, the anode is maintained at a high positive potential with respect to the cathode for the purpose of establishing a strong electrostatic field between the photocathode and the imaging screen.
In operation, photons of radiant energy emanating from localized areas of an external object pass through the input faceplate of the image intensifier tube and impinge on corresponding localized areas of the photocathode. As a result, the photocathode emits an equivalent electron image which is accelerated by the strong electrostatic field toward the imaging screen assembly at the other end of the tube. The accelerated electron image, thus amplified, impinges on the phosphor layer of the imaging screen assembly with sufficient kinetic energy to produce a corresponding visual image which may be viewed through the output faceplate of the tube.
Image intensifier tubes have been developed for converting faint visible light images directly into bright visible images. However, similar attempts to develop an image intensifier tube which converts infrared radiational images directly into bright visible images have not been too successful. Because of the comparatively low energy quanta associated with infrared wavelengths, a two transitional energy level technique generally is employed in image intensifier tubes of the prior art. Thus, electrons in the photocathode material usually are excited to higher energy levels by an auxiliary source of radiation, commonly referred to as the "pumping" source. In this manner, an infrared radiational image impinging on the photocathode can raise the excited electrons up to the energy level required for producing a corresponding visible light image. However, the described method requires two radiational sources to produce the desired result and, consequently, is highly inefficient and expensive. Therefore, there is a definite need for an image intensifier tube which can convert infrared radiational images directly into visible light images without the aid of a "pumping" source of radiation.